The invention relates to a device for skid control for a vehicle with electromotive vehicle drive according to the preamble of patent claim 1.
Devices for anti-slip control (ASC) have the purpose to prevent slip of the drive wheels during acceleration. The goal is on one hand to achieve an acceleration for the vehicle which is as efficient as possible and on the other hand to not unnecessarily adversely affect the stability of the vehicle during acceleration.
When the wheel slip becomes excessive there are generally two options to intervene by means of an anti-slip control. On one hand the motor torque can be reduced via the motor control when for example the drive axle as a whole has an excessive slip. However, when the slip is only excessive on one drive wheel or when the slip is too great, the slip can be reduced by braking. However, anti-slip control by means of breaking has the disadvantage that the drive wheels are often decelerated excessively thus resulting in loss of traction potential.
Electromotive vehicle drives that use electric machines as drive motors and can be operated in the motor mode as well as in the generator mode, have a rotational torque which is generated by the Lorentz force and has a positive value for acceleration and a negative value for deceleration. The Lorentz force is proportional to the cross product of the current in the stator and the magnetic flux, which permeates the stator and the rotor. Owing to this circumstance, the rotational torque in an electric machine cannot only be changed by changing the current and the magnetic flux but also by the angle formed between the two vector values. During control of electric machines the voltage at the clamps of the machine is therefore changed so that the rotational torque resulting from the cross product assumes the desired value. In which manner the three values can be adjusted or adapted strongly depends on the respective type of machine. In rotating-field machines, the current vector rotates in the stator with the same speed as the magnetic field (rotational field) for generating a rotational torque. In synchronous machines, the direction of the rotational field is predetermined by its construction. In order to generate a constant force, the rotor therefore has to rotate with the same speed at which the current in the stator rotates. When the current leads the field, a positive rotational torque (acceleration) results and when it lags the field a negative torque (braking) results.
From WO 97/08008 and anti-slip control system for a motor vehicle using an electric machine is known in which a rotational torque is predetermined for the anti-slip control (ASC) of the electric machine. The anti-slip control only becomes active when it is detected that a threshold value of the drive slip is exceeded.
From DE 197 32 460 A1 and method for controlling a multi-motor drive with a common converter for controlling the drive motors is known. In this known method the output frequency of the converter is adapted to the rotational speed of the drive wheels, which have a good contact to the street. This is intended to prevent a runaway of the output frequency of the converter during acceleration processes due to the slip of at least one drive wheel. The drive wheels that have good contact to the street then ensure an acceleration of the vehicle.
The invention is based on the object to create a device for anti-slip control for a vehicle with electromotive vehicle drive with which good acceleration values for the vehicle can be achieved.